Microscope objective lenses having a large numerical aperture are advantageous in improving resolution and brightness for observation using fluorescence. However, large numerical apertures hamper the forming of flat images. One of the lens types that facilitates forming flat images is a set of meniscus-shaped optical elements that hereinafter will be termed a Gauss lens set. Each meniscus-shaped optical element of a Gauss lens set may be formed of a positive lens element and a negative lens element that are cemented to form a single lens so that, overall, the lens is meniscus-shaped. One of these meniscus-shaped optical elements has its concave surface on the image side and the other has its concave surface on the object side so that the two concave surfaces are adjacent and face each other. Such a Gauss lens set can compensate for a positive Petzval sum (created by positive lenses) using the negative Petzval sum created by the negative refractive power of the facing concave surfaces of the meniscus-shaped optical elements, thereby controlling the curvature of field to ensure flat images.
Microscope objective lenses comprising a Gauss lens set having a large aperture and that facilitate the forming of flat images are described in the prior art. For example, Japanese Laid Open Patent Application No. H8-136816 has a Gauss lens set that provides a magnification of 20xc3x97 and uses liquid immersion to achieve a numerical aperture of 0.8. Japanese Laid Open Patent Application No. H10-274742 has a Gauss lens set that provides a magnification of 40xc3x97 and uses liquid immersion to achieve a numerical aperture of 1.3, and Japanese Laid Open Patent Application No. H7-35983 has a Gauss lens set that provides a magnification of 60xc3x97 and uses liquid immersion to achieve a numerical aperture of 1.4. As in the prior art described above, a Gauss lens set is often used to ensure a flat image with an objective lens having a low magnification and a high numerical aperture, or with an objective lens that uses liquid immersion to achieve a high magnification and an even higher numerical aperture. However, there are limitations in ensuring a flat image with a low magnification using a lens having still higher numerical apertures or in obtaining flatter images using a lens having still higher magnifications.
The object of the present invention is to provide a microscope objective lens having a large numerical aperture that ensures satisfactorily corrected, flat images.